The fundamental of face image recognition is verifying whether two compared faces belong to the same identity or not based on biological features. Recognized with other traditional means of recognition, such as fingerprint recognition, the face image recognition may be accurate, easy-to-use, difficult to counterfeit, cost-effective and non-intrusive, and thus it is widely used for safety applications. Face image recognition has been extensively studied in recent decades. Existing methods for face image recognition generally comprises two steps: feature extraction and recognition. In the feature extraction stage, a variety of hand-crafted features are used. More importantly, the existing methods extract features from each face image separately and compare them later at the face recognition stage. However, some important correlations between the two compared face images may have been lost at the feature extraction stage.
At the recognition stage, classifiers are used to classify two face images as having the same identity or not, or other models are employed to compute the similarities of two face images. The purpose of these models is to separate inter-personal variations and intra-personal variations. However, all of these models have shallow structures. To handle large-scale data with complex distributions, large amount of over-completed features may need to be extracted from the faces. Moreover, since the feature extraction stage and the recognition stage are separate, they cannot be jointly optimized. Once useful information is lost in the feature extraction stage, it cannot be recovered in the recognition stage.